Flexible Organic Field-effect Transistors Research Articles

Chemically tunable ultrathin silsesquiazane interlayer for n-type and p-type organic transistors on flexible plastic.

In organic field-effect transistors (OFETs), surface modification of the gate-dielectric is a critical technique for enhancing the electrical properties of the device. Here, we report a simple and versatile method for fabricating an ultrathin cross-linked interlayer (thickness ∼3 nm) on an oxide gate dielectric by using polymeric silsesquiazane (SSQZ). The fabricated siloxane film exhibited an ultrasmooth surface with minimal hydroxyl groups; the properties of the surface were chemically tuned by introducing phenyl and phenyl/fluorine pendent groups into the SSQZ. The growth characteristics of two semiconductors-pentacene (p-type) and N,N'-ditridecyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C13, n-type)-on this ultrathin film were systematically investigated according to the type of pendent groups in the SSQZ-treated gate dielectric. Pentacene films on phenyl/fluorine groups exhibited large grains and excellent crystalline homogeneity. By contrast, PTCDI-C13 films exhibited greater crystalline order and perfectness when deposited on phenyl groups rather than on phenyl/fluorine groups. These microstructural characteristics of the organic semiconductors, as well as the dipole moment of the pendent groups, determined the electrical properties of FETs based on pentacene or PTCDI-C13. Importantly, compared to FETs in which the gate dielectric was treated with a silane-coupling agent (a commonly used surface treatment), the FETs fabricated using the tunable SSQZ treatment showed much higher field-effect mobilities. Finally, surface treatment with an ultrathin SSQZ layer was also utilized to fabricate flexible OFETs on a plastic substrate. This was facilitated by the facile SSQZ deposition process and the compatibility of SSQZ with the plastic substrate.

Flexible, ambipolar organic field-effect transistors based on the solution-processed films of octanaphthoxy-substituted bis(phthalocyaninato) europium

A new sandwich-type bis(phthalocyaninato) europium complex [Pc(OAr)8]Eu[Pc(OAr)8] [Pc(OAr)8 = 2,3,9,10,16,17,23,24-octanaphthoxy phthalocyanine], was synthesized and fabricated into organic field-effect transistors devices on a flexible plastic substrate by using a simple solution-based quasi–Langmuir–Shäfer method. Ambipolar transport has been obtained in [Pc(OAr)8]Eu[Pc(OAr)8]-based thin film transistors with the carrier mobilities to 0.10 cm2 V−1 s−1 for holes and 0.01 cm2 V−1 s−1 for electrons, respectively. The energy levels of the highest occupied molecular orbital (−4.63 eV) and the lowest unoccupied molecular orbital (−3.41 eV) were obtained based on electrochemical studies of this double-decker complex. These values just simultaneously meet the energy levels required for p-type and n-type natures of organic semiconductors, suggesting the ambipolar organic semiconducting nature of this double-decker complex. Organized film microstructures and J-type molecular stacking mode for the double-decker complex in the quasi–Langmuir–Shäfer film are revealed. This study constitutes the first attempt to fabricate low-cost, flexible ambipolar phthalocyanine-based organic field-effect transistors by using simple solution-processing methods.

A hydrogel capsule as gate dielectric in flexible organic field-effect transistors

A jellified alginate based capsule serves as biocompatible and biodegradable electrolyte system to gate an organic field-effect transistor fabricated on a flexible substrate. Such a system allows operating thiophene based polymer transistors below 0.5 V through an electrical double layer formed across an ion-permeable polymeric electrolyte. Moreover, biological macro-molecules such as glucose-oxidase and streptavidin can enter into the gating capsules that serve also as delivery system. An enzymatic bio-reaction is shown to take place in the capsule and preliminary results on the measurement of the electronic responses promise for low-cost, low-power, flexible electronic bio-sensing applications using capsule-gated organic field-effect transistors.

Electronic readout enzyme-linked immunosorbent assay with organic field-effect transistors as a preeclampsia prognostic.

Organic field-effect transistor (OFET) sensors can meet the need for portable and real-time diagnostics. An electronicreadout enzyme-linked immunosorbent assay using OFETs for the detection of a panel of three biomarkers in complex media to create a pre-eclampsia prognostic is demonstrated, along with biodetection utilizing a fully inkjet-printed and flexible OFET to underscore our ability to produce disposable devices.

Directly drawn poly(3-hexylthiophene) field-effect transistors by electrohydrodynamic jet printing: improving performance with surface modification.

In this study, direct micropatterning lines of poly(3-hexylthiophene) (P3HT) without any polymer binder were prepared by electrohydrodynamic jet printing to form organic field-effect transistors (OFETs). We controlled the dielectric surface by introducing self-assembled monolayers and polymer thin films to investigate the effect of surface modifications on the characteristics of printed P3HT lines and electrical performances of the OFETs. The morphology of the printed P3HT lines depended on the surface energy and type of substrate. The resulting OFETs exhibited high performance on octadecyltrichlorosilane-modified substrates, which was comparable to that of other printed P3HT OFETs. In order to realize the commercialization of the OFETs, we also fabricated a large-area transistor array, including 100 OFETs and low-operating-voltage flexible OFETs.

Polyol synthesis of silver nanostructures: Inducing the growth of nanowires by a heat-up process

The polyol synthesis of silver (Ag) nanostructures typically involves the rapid injection of Ag precursor to a preheated, ethylene glycol solution containing polymeric stabilizers and other additives. Here we report that Ag nanowires can be synthesized in high yields by applying a heat-up process in the polyol synthesis. Electron microscopy studies revealed that multiple-twinned Ag seeds were generated preferentially during the heat-up procedure, and then grew into nanowires. We also demonstrate that these Ag nanowires can be applied as electrode materials for the fabrication of flexible and transparent organic field-effect transistors with a reasonably high hole-mobility.

Cellulose nanocrystals thin films as gate dielectric for flexible organic field-effect transistors

We report on the fabrication of flexible field-effect transistor using poly[N-9”-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] as active channel and cellulose nanocrystals (CNC) as gate dielectric on indium tin-oxide coated polyethyleneterephthalate substrate. The use of CNC as gate dielectric material was found favorable for carrier transport at the semiconductor/dielectric interface with the device showing an on–off ratio of ≈104. These results indicate that CNC is a promising gate dielectric layer for flexible organic field effect transistors.

Very facile fabrication of aligned organic nanowires based high-performance top-gate transistors on flexible, transparent substrate

Aligned single-crystalline organic nanowires (NWs) show promising applications in flexible and stretchable electronics, while the use of pre-existing aligned techniques and well-developed photolithography techniques are impeded by the large incompatibility with organic materials and flexible substrates. In this work, aligned copper phthalocyanine (CuPc) organic NWs were grown on flexible and transparent poly(dimethylsiloxane) (PDMS) substrate via a grating-assisted growth approach. Furthermore, a simple yet efficient etching-assisted transfer printing (ETP) method was used to achieve CuPc NW array-based flexible top-gate organic field-effect transistors (OFETs) with a high mobility up to 2.0cm2V−1s−1, a small operating voltage within ±10V, a high on/off ratio>104, and excellent bend stability with bending radius down to 3mm. It is expected that the high-performance organic NW array-based top-gate OFETs with exceeding bend stability will have important applications in future flexible electronics.

Multilevel Nonvolatile Flexible Organic Field‐Effect Transistor Memories Employing Polyimide Electrets with Different Charge‐Transfer Effects

The electrical memory characteristics of the n-channel organic field-effect transistors (OFETs) employing diverse polyimide (PI) electrets are reported. The synthesized PIs comprise identical electron donor and three different building blocks with gradually increasing electron-accepting ability. The distinct charge-transfer capabilities of these PIs result in varied type of memory behaviors from the write-one-read-many (WORM) to flash type. Finally, a prominent flexible WORM-type transistor memory is demonstrated and shows not only promising write-many-read-many (WMRM) multilevel data storage but also excellent mechanical and retention stability.

Strain‐Tunable Superconducting Field‐Effect Transistor with an Organic Strongly‐Correlated Electron System

A novel type of flexible organic field-effect transistor in which strain effects can be finely tuned continuously has been fabricated. In this novel device structure, electronic phases can be controlled both by "band-filling" and by "band-width" continuously. Finally, co-regulation of "band-filling" and "band-width" in the strongly-correlated organic material realize field-induced emergence of superconducting fractions at low temperature.

Flexible organic field-effect transistors based on electrospun conjugated polymer nanofibers with high bending stability

We report on flexible, single electrospun nanofiber field-effect transistors made by a blend of poly(3-decylthiophene) and poly(3-hexylthiophene), assessing for the first time the performances of this class of devices in terms of stability upon repeated tensile bending. Charge-carrier mobilities in the nanofiber-based device are estimated of the order of 10−3cm2/(Vs). Repeated cycles of bending and relaxing are performed, and the evolution of the device current–voltage characteristics is monitored up to 1000 cycles. We find that during bending the mobility is higher than that measured in planar conditions, and that after about 100 bending cycles it rapidly stabilizes. The here observed bending stability suggests a high compatibility of electrospun nanofibers with devices fabricated by roll-to-roll processes, and with bendable or wearable electronics.

The application of orthogonal photolithography to micro-scale organic field effect transistors and complementary inverters on flexible substrate

Micro-scale pentacene organic field effect transistors (OFETs) were fabricated on a flexible poly(ethylene terephthalate) (PET) substrate. By applying a highly fluorinated developing solvents and its compatible photoresist materials, it has become possible to make the micro-scale patterning for organic devices using standard photolithography without damaging the underlying polymer layers. The flexible pentacene OFETs with 3 μm-sized channel length exhibited stable electrical characteristics under bent configurations and under a large number of repetitive bending cycles. Furthermore, we demonstrated micro-scale organic complementary inverters on a flexible PET substrate using p-type pentacene and n-type copper hexadecafluorophthalocyanine materials.

Fabrication of ultra-flexible, ultra-thin organic field-effect transistors and circuits by a peeling-off method

We present a new and simple way of fabricating ultra-thin and flexible organic field-effect transistors and circuits by directly peeling them off from conventional SiO2 substrates. The devices show excellent flexibility up to a bending radius of 1 mm. Free-standing OFETs as thin as about 600 nm can also be made in this way. In addition, flexible complementary inverters with a gain of 100 can be fabricated by the peeling-off method.

Aligned nanowire arrays on thin flexible substrates for organic transistors with high bending stability

Flexible organic field-effect transistors (OFETs) are fundamental elements for the development of rollable displays, flexible circuits, and bio-compatible sensors. Although single-crystalline organic nanostructures show great promise for the construction of high-performance OFETs, their applications in flexible devices are rarely investigated due to the large incompatibility with existing processing techniques. Here, we report the fabrication of copper phthalocyanine (CuPc) nanowire (NW) array-based top-gate OFETs on flexible PDMS substrates via an efficient and versatile etching-assisted transfer printing (ETP) method. The thickness dependent bending stability of the OFETs was investigated. It was demonstrated that the device with a thinner substrate thickness possessed a higher bending stability owing to the smaller bending-induced mechanical strain. Also, the bending direction played an important role in determining the bending stability; a higher bending stability was achieved when the bending direction was perpendicular to the NW length direction. We also performed a real-life flexibility test by repeatedly crumpling the device in the palm of the hand, verifying the excellent bending stability and reproducibility of the flexible devices. It is expected that the organic NW array-based top-gate OFETs with high bend stability will have important applications in future flexible electronics.

Progress of flexible organic non-volatile memory field-effect transistors

Flexible organic non-volatile memory field-effect transistors (ONVMFETs) are promising candidates in the field of flexible organic electronic devices, which can be used in flexible radio frequency tags, memories, integrated circuits and large-area displays, because of their remarkable advantages such as flexibility, lightweight, low cost and large-area organic electronics. On the basis of the introduction of the development of flexible ONVMFETs in terms of substrates, structures and characteristics, the classification of flexible ONVMFETs is summarized. Meanwhile, we discuss the effects of mechanical stress and temperature on the performance of flexible ONVMFET. Finally, some prospects as well as the challenges are pointed out.

Solution Processable High Dielectric Constant Nanocomposites Based on ZrO2 Nanoparticles for Flexible Organic Transistors

A solution-based strategy for fabrication of high dielectric constant (κ) nanocomposites for flexible organic field effect transistors (OFETs) has been developed. The nanocomposite was composed of a high-κ polymer, cyanoethyl pullulan (CYELP), and a high-κ nanoparticle, zirconium dioxide (ZrO2). Organic field effect transistors (OFETs) based on neat CYELP exhibited anomalous behavior during device operation, such as large hysteresis and variable threshold voltages, which yielded inconsistent devices and poor electrical characteristics. To improve the stability of the OFET, we introduced ZrO2 nanoparticles that bind with residual functional groups on the high-κ polymer, which reduces the number of charge trapping sites. The nanoparticles, which serve as physical cross-links, reduce the hysteresis without decreasing the dielectric constant. The dielectric constant of the nanocomposites was tuned over the range of 15.6-21 by varying the ratio of the two components in the composite dielectrics, resulting in a high areal capacitance between 51 and 74 nF cm(-2) at 100 kHz and good insulating properties of a low leakage current of 1.8 × 10(-6) A cm(-2) at an applied voltage of -3.5 V (0.25 MV cm(-1)). Bottom-gate, top-contact (BGTC) low operating voltage p-channel OFETs using these solution processable high-κ nanocomposites were fabricated by a contact film transfer (CFT) technique with poly(3-hexylthiophene) (P3HT) as the charge transport layer. Field effect mobilities as high as 0.08 cm(2) V(-1) s(-1) and on/off current ratio of 1.2 × 10(3) for P3HT were measured for devices using the high-κ dielectric ZrO2 nanocomposite. These materials are promising for generating solution coatable dielectrics for low cost, large area, low operating voltage flexible transistors.

Flexible air-stable three-dimensional polymer field-effect transistors with high output current density

Flexible air-stable short-channel polymer organic field-effect transistor (OFET) arrays with high saturated output current density are demonstrated by utilizing a novel solution-processed naphthobisthiadiazole (NTz) based donor–acceptor semiconducting polymer (PNTz4T) and designing a three-dimensional vertical channel structure with an extremely large ratio of channel width to channel length. The saturated mean field-effect mobility of 0.16cm2/Vs of the short-channel polymer devices remains over one month resulting in air-stable OFET arrays with high on/off ratio over 106 and powerful current–density exceeding 0.3A/cm2 under low operation voltage, both of which meet the requirements for such applications as driving organic light-emitting diodes in active-matrix displays.

Study of carrier transport in flexible organic field-effect transistors: Analysis of bending effect and microscopic observation using electric-field-induced optical second-harmonic generation

We studied the effect of bending on the carrier transport properties of flexible 6,13-Bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) field-effect transistors. Results showed that the effective carrier mobility increased ~30% with a 1.5% mechanical strain (compressive stress), whereas it decreased ~15% with a −1.5% strain (tensile stress). Theoretical analysis based on the Maxwell–Wagner model was carried out, and suggested that both carrier mobility and carrier density in the organic field-effect transistor (OFET) channel were modulated due to the mechanical strains. The microscopic electric-field-induced second harmonic generation (EFISHG) images showed that carrier transport was governed by the presence of grains of TIPS-pentacene. The EFISHG observation is a powerful tool to investigate carrier transport in flexible OFETs which are being subjected to mechanical strains.

Flexible organic field-effect transistor fabricated by thermal press process

High throughput printing processes have been developed for the fabrication of organic flexible sheet devices. However, printing processes inevitably use toxic solvents to prepare inks of organic semiconductors. Here we propose a novel and complementary method for the preparation of flexible sheet electronics. A thermal pressing method has several advantages, such as (i) being solvent-free, (ii) resulting in flexible device with structural durability against bending, (iii) making simultaneous sealing available to prevent physical or atmospheric damage, and (iv) providing seamless application to roll-to-roll processes by combination with toner technology. We have demonstrated that the characteristics of a flexible organic field-effect transistor (OFET) fabricated by the thermal press process were not degraded during or after the bending test until a bending radius of 2 mm was achieved, which is sufficient for the concept of a rolled-up type flexible sheet display.

Highly Transparent and Flexible Nanopaper Transistors

Renewable and clean "green" electronics based on paper substrates is an emerging field with intensifying research and commercial interests, as the technology combines the unique properties of flexibility, cost efficiency, recyclability, and renewability with the lightweight nature of paper. Because of its excellent optical transmittance and low surface roughness, nanopaper can host many types of electronics that are not possible on regular paper. However, there can be tremendous challenges with integrating devices on nanopaper due to its shape stability during processing. Here we demonstrate for the first time that flexible organic field-effect transistors (OFETs) with high transparency can be fabricated on tailored nanopapers. Useful electrical characteristics and an excellent mechanical flexibility were observed. It is believed that the large binding energy between polymer dielectric and cellulose nanopaper, and the effective stress release from the fibrous substrate promote these beneficial properties. Only a 10% decrease in mobility was observed when the nanopaper transistors were bent and folded. The nanopaper transistor also showed excellent optical transmittance up to 83.5%. The device configuration can transform many semiconductor materials for use in flexible green electronics.